GB2177214A - Non-polarizable electrode - Google Patents

Abstract

A non-polarizable electrode for use with an induced polarization logging tool. The electrode comprises a closed housing 10 having a porous wall portion 11, an electrode 20 of a metal forming a slightly soluble chloride and having a chloridised end, and a saturated electrolyte of KCl and the metal chloride. The electrode is sealed in the housing. The housing may be flexible to equalize fluid pressure across the frit and prevent poisoning of the electrode by mud components. Preferably the metal is silver but cadmium, zinc and thallium may be used. The end of the electrode 30 or a rod 33 inserted therein may be formed as a male connector. <IMAGE>

Description

SPECIFICATION Non-polarizable electrode The present invention relates to a non-polarizable electrode that is particularly useful in induced polarization logging tools. In addition the electrode is useful in any electrical measuring circuit where a non-polarizable electrode is necessary.

In induced polarization logging a sonde containing a plurality of electrodes is lowered in a borehole to measure the induced polarization of the formation surrounding the borehole. In particular, a current electrode is used to pass an alternating current through the formation and a plurality of voltage electrodes are used to measure the in-phase and out-of-phase voltage that is developed within the formation.

Such an induced polarization logging tool is described in U.S. Patent No. 4,359,687.

Since the induced polarization response of earth formations is very small, extreme accuracy is required in induced polarization logging.

In particular, any electrical polarization of the voltage electrodes will seriously distort the phase measurements obtained with an induced polarization logging tool.

The prior art suggests that lead electrodes be used for measuring the signals in induced polarization logging tools. For example, in an article by Snyder et al entitled "Complex Formation Resistivity The Forgotten Half of the Resistivity Log", SPWLA 18th Annual Logging Symposium, June 5-8, 1977, the use of lead electrodes in induced polarization logging tools is described. Lead electrodes, however, are polarizable and introduce considerable error in the induced polarization measurements. The polarization becomes increasingly large as the measuring frequency is lowered. Moreover, lead electrodes can have large asymmetry potentials in contact with certain drilling muds which prevents the use of direct-coupled amplifiers in an induced polarization tool such as that described in U.S. Patent No. 4,359,687.

The present invention solves these problems by providing a non-polarizable electrode for induced polarization logging tools that can be used in a borehole at high temperatures and high pressures.

The electrode according to the invention comprises: a closed housing, said housing having a portion of its wall formed of a porous material; an electrode member, said electrode member being formed from a metal that forms a chloride salt that is only slightly soluble in water, said electrode being chloridized to form a chloride salt on the surface of said electrode, said electrode being disposed in said housing with at least a portion of the electrode projecting out of said housing; sealing means, said sealing means being disposed to seal the portion of said electrode that projects out of said housing; and a saturated solution of the chloride salt of the metal forming said electrode and potassium chloride, said housing being filled with said solution.

In a preferred embodiment of the invention the electrode member is a silver rod which has been chloridized to form a silver chloride coating, immersed in a saturated silver chloride solution. In this case, the silver chloride is the chloride salt.

Preferably the non-polarizable electrode is mounted in a flexible electrode housing with one end of the housing being closed by a porous frit-type member. The housing is filled with saturated KCI and AgCl electrolytes so that the electrode will respond to the potential appearing in the borehole fluid. A small amount of solid KCI and AgCl is also placed inside the housing to ensure saturated conditions at any temperature. The other end of the housing may be formed by the male half of an electrical connector with the female portion being mounted on the logging tool. This permits the electrodes to be easily replaced on the logging tool and removed for storage after completion of the logging run.

The arrangement of a flexible electrode housing with porous frit allows the brine inside the housing to equalize pressure with the borehole fluid. In addition, the frit impedes the transport of contaminants in the borehole fluid which might poison the silver/silver chloride electrodes. The housing and frit also serves to maintain a saturated KCI and AgCl environment around the electrode. Still another advantage is that the electrode is not in the direct current path in the borehole, which further reduces the possibility of electrode polarization.

An advantage of the non-polarizable electrodes according to the invention is that they have extremely low asymmetry potentials which allows the amplifiers in the induced polarization logging tool to be decoupled and also enables interchangeability between electrodes. Thus, any of these electrodes can be placed in any connector on the logging tool.

A further advantage of the electrodes according to the invention is that these electrodes are stable, sensitive, and reproducible.

They are exceptionally durable and can be operated at the high temperatures and high pressures found in the borehole environment. They are also in-expensive and easy to manufacture.

They can be used in any orientation. For example, their compact size allows these electrodes to be easily mounted on a pad or in suitable recesses formed on the exterior of the logging sonde.

The present invention will be more easily understood from the following description of a preferred embodiment when taken in conjunction with the attached drawing in which: Figure 1 shows an elevation view of an electrode constructed according to this invention.

Referring now to Figure 1, there is shown an electrode which is enclosed by housing 10.

Housing 10 may be constructed of any electrically insulated material with sufficient flexibility but is preferably VESPEL (trade mark) manufactured by DuPont Chemical Company of Wilmington, Delaware. VESPHL is preferred for its durability, high-temperature stability, machinability, and surface adhesion which enables a strong epoxy bond.

The left hand end of the housing is closed by a porous frit member 11 which seats against the shoulder 12 machined on the inner diameter of the housing 10. The porous frit member 11 may be formed from any inert porous material, but preferably from porous porcelain such as that manufactured by Coors Porcelain Co. of Golden, Colorado. Preferably, the porous frit member 11 is sealed in the housing shoulder 12 by a suitable adhesive, for example epoxy, applied around its circumference. The porous frit member 11 is chosen to allow only slow fluid transport through the frit but still have good electrical continuity. Frit material such as Coors' unglazed porcelain plate P-1/2 B-C with a porosity of about 40% and a permeability of about 1 millidarcy satisfies these requirements.

In operation in the borehole, the borehole fluid pressure squeezes the housing 10 which is compressed sufficiently to transmit this pressure to the KCI + AgCl solution within the housing. This immediately equalizes the fluid pressure across the frit so that no net flow of borehole fluid occurs into the housing.

Ionic diffusion through the frit still occurs, but the frit is chosen to be sufficiently tight that the total amount of diffusion is negligible during the time the logging tool is in the borehole. This lowers the possibility of poisoning the Ag-AgCI electrode with contaminants in the drilling mud, such as sulphide ions.

The electrode 20 is formed from a solid rod of silver which is provided with a spherical end. The silver rod is 99.99+% purity and commercially available from distributors such as Engeihard Metals of New Jersey. The outer surface of the electrode should be provided with a polished finish so there will be no voids in the silver chloride coating when it is chloridized, then annealed at 900"C for 24 hours. The electrode 20 is provided with a centre bore 23 in which the male half of the electrical connector 30 is sealed. Preferably, the electrode 20 is a press-fit on the centre lead 33. Alternatively, the centre lead 33 can be soldered into the centre bore 23.

The centre lead 33 is part of an electrical connector 30 which may take various forms.

Preferably, the electrical connector is a highpressure connector sold under the trade name KEMLON (trade mark) manufactured by Keystone Engineering Company of Houston, Texas.

These connectors are coated with hard insulating material which can withstand high temperatures and high pressures and are used extensively in logging tools. The connector is provided with a threaded housing 32 and an extension 31 that threads into a female thread machined in the housing 10. In particular, the silver electrode 20 projects through an opening 21 formed in a wall of the housing with an O-ring 22 being provided for sealing the electrode in the housing. The O-ring 22 is compressed by the extension 31 on the connector as it is threaded into the housing. The threads in housing 10 allows the housing to be unscrewed from the connector 30 for refilling the housing chamber with fresh electrolyte solution.The connector 30 also has a flexible ring 34 integrally mounted for forming a pressure-tight seal with a female KEMLON connector (not shown) which is, in turn, connected through the wall of the induced polarization logging tool.

The silver electrode is chloridized before it is placed in the housing. In particular, the electrode is placed in a glass dish filled with the plating solution of 0.1 M potassium chloride of a reagent grade. The cathode is a platinum screen preferably placed in the centre of the dish and surrounded by a plurality of equallyspaced silver electrodes. The plating current should be approximately 5 milliamperes per square centimetre of electrode surface area.

Thus, for six electrodes made from rod 0.3 cm diameter by 2.5 cm long, the plating current is approximately 6 milliamperes per electrode or 36 milliamperes in total. A magnetic stirrer in the bottom of the plating dish provides more uniform plating. Alternatively, an ultrasonic bath can be used.

As the plating progresses and gas bubbles appear on the electrode surface, the current should be gradually lowered and the glass dish agitated to loosen bubbles that have adhered to the surface. Chloridizing is terminated when the resistance per electrode measured between the cathode and anode begins to increase substantially. For the six electrode system disclosed hereinbefore, the chloridizing is terminated when the resistance per electrode is greater than approximately 850 ohms. The resistance per electrode can be determined by measuring the supply current and voltage and dividing the current per electrode by the total voltage.

Other modes of preparation of the silver chloride surface are possible, such as thermal techniques based on decomposition in a furnace of a paste of AgO and AgCl03, or thermal/electrolytic techniques where electrolytic formation of AgCl is performed on silver thermally deposited on a platinum substrate, or miscellaneous silver chloride precipitation techniques. These methods are described in detail by Ives and Janz in the text "Reference electrodes", Academic Press, New York (1961).

After the chloridizing operation, the electrodes are electrically shorted together and stored for at least 15 hours aging in dilute KCI solution. For long term storage the electrodes are stored in the dark in individual containers which are filled with saturated KCI solution which is also saturated with AgCl. The electrodes are electrically shorted together during long-term storage to reduce asymmetry potentials. The asymmetry potentials of these electrodes is typically 100 microvolts or less.

The housing 10 is filled with saturated KCI solution that is also saturated with AgCl. The use of potassium chloride solution minimizes junction potentials at the drilling mud junctions because the transport numbers for potassium and chloride are both approximately equal to 0.5. Use of saturated KCI solution eliminates the need for careful concentration measurements at the well site. Moreover, the high electrical conductivity of the saturated KCI solution improves the electrical contact to the borehole fluid. Yet another advantage of using saturated KCI is the minimization of electrode poisoning by sulphide, bromide or other ions in the borehole since the large population of potassium and chloride ions in KCI solution inhibits the adsorption of the contaminant ions on the silver surface.

The solution housing 10 is also saturated with respect to AgCl to prevent dissolution of the AgCl plating in the saturated KCI. In addition, the saturated AgCl solution provides a constant activity of silver chloride in solution and thus a constant reference potential for the electrode at a given temperature.

The storage of the electrodes in the same solution that is used while logging the borehole ensures that the characteristics of the electrodes will remain constant. This is desirable since it reduces the adjustments that are required when electrodes are changed.

The electrodes are normally mounted in recesses formed on the exterior surface of the logging tools.

In an alternative embodiment, the silver/silver chloride electrode can be utilized without the VESPEL chamber, although this increases the possibility of poisoning the electrode with borehole contaminants.

Other types of reversible electrodes of the second kind are also included within the scope of this invention, for example zinc/zinc chloride, cadmium/cadmium chloride, and thallium/thallium chloride. None of these electrodes are as stable as silver/silver chloride and their use is also more limited due to greater solubility under many conditions. Similarly, other silver halides can be used besides silver chloride. Silver/silver chloride is preferred, however, due to the predominance of chloride ions in the borehole mud.

Still another alternative embodiment would replace the saturated KCI and AgCl solution with a suitable gel, such as Agar-Agar, which is saturated with KCI and AgCl. The use of a gel serves to further minimize fluid and ion exchange with the borehole mud. The Agar Agar gel mixture is made starting with saturated KCI and AgCl brine, then mixing in 3% Agar-Agar powder. The solution is heated to dissolve the Agar-Agar, stirred gently, and allowed to cool and gel.

Yet another alternative embodiment would be to replace the silver rod in Figure 1 with an electrode constructed of sintered porous silver. Although requiring more steps in fabrication, this embodiment provided much greater surface area and protects the silver chloride coating by having the majority of the coating on the interior porous spaces. The sintered porous silver is made from fine grains of silver which are compacted together and sintered in a reducing gas environment. The sintered porous silver is then chloridized as described herein before.

In an alternative embodiment, the silver cylindrical rod electrode is replaced by a diskshaped electrode, such as would be used in a pad-type induced polarization logging tool. The low profile of this electrode design allows a plurality of electrodes to be arranged in the pad which is then forced against the wall of the borehole.

Still another embodiment is to replace the chloridized silver rod of Figure 1 with coiled chloridized silver wire. The coiled silver wire can also be used in a pad-type logging tool with the electrodes arranged as concentric rings.

Many other variations and modifications may be made in the apparatus and techniques hereinbefore described by those having experience in this technology without departing from the concept of the present invention. Accordingly, it should be clearly understood that the apparatus depicted in the accompanying drawing and referred to in the foregoing description is illustrative only and is not intended as a limitation on the scope of this invention.

Claims (12)

1. A non-polarizable electrode comprising: a closed housing, said housing having a portion of its wall formed of a porous material; an electrode member, said electrode member being formed from a metal that forms a chloride salt that is only slightly soluble in water, said electrode being chloridized to form a chloride salt on the surface of said electrode, said electrode being disposed in said housing with at least a portion of the electrode projecting out of said housing; sealing means, said sealing means being disposed to seal the portion of said electrode that projects out of said housing; and a saturated solution of the chloride salt of the metal forming said electrode and potassium chloride, said housing being filled with said solution.

2. The non-polarizable electrode of claim 1 wherein said metal is silver.

3. The non-polarizable electrode of claim 1 wherein said housing is cylindrical and said porous material forms one end wall of said cylindrical housing.

4. The non-polarizable electrode of claim 3 wherein said electrode member has a solid rod shape.

5. The non-polarizable electrode of claim 3 wherein said electrode member is a sintered porous silver rod.

6. The electrode of claim 1 wherein said housing is filled with a saturated solution of KCI and AgCI.

7. The non-polarizable electrode of claim 1 wherein said metal is selected from cadmium, zinc and thallium.

8. The electrode of claim 1 wherein said electrode member is a solid cylindrical member.

9. The electrode of claim 8 wherein the sealing means comprises an "O"-ring seal surrounding the electrode member.

10. The electrode of claim 9, further comprising a pressure resistant fluid-tight electrical connector, said electrical connector being mounted in said housing and electrically coupled to said one end of said electrode member, wherein said electrode member is provided with a central bore, said electrical connector including a pin-type central electrode, said pintype electrode being electrically and mechanically secured in the bore of the said electrode.

11. The electrode of claim 1, wherein the electrode is mounted in an induced polarization logging tool.

12. A non-polarizable electrode according to claim 1, substantially as described hereinbefore with reference to the accompanying drawing.